For the past 50 years, perovskite Pb(ZrxTi1-x)O3 (PZT) piezoelectric ceramics have dominated the commercial market of electronic devices, including piezoelectric sensors, actuators and medical ultrasonic transducers, due to their high piezoelectric and electromechanical coupling factors. For example, the shear piezoelectric coefficient d15 and electromechanical coupling factor k15 for PZT5A type (DOD Type II) materials are found to be on the order of about 400 pC/N and approximately 70%, respectively. Innovations in electronic devices have been the driving force for new developments in piezoelectric materials, including relaxor-PT single crystals.
The excellent piezoelectric properties of relaxor-PT single crystals, including Pb(Mg1/3Nb2/3)O3—PbTiO3 (“PMN-PT”) and Pb(In0.5Nb0.5)O3—Pb(Mg1/3Nb2/3)O3—PbTiO3 (“PIN-PMN-PT”), have attracted considerable interest over the last decade, particularly for applications in high performance medical transducers. However, their commercial use has been limited due to high variation of the dielectric and piezoelectric properties with temperature. Furthermore, the low coercive field of current relaxor-PT crystals further limits their application.
Single crystal compositions near their respective morphotropic phase boundaries (MPB) exhibit longitudinal piezoelectric coefficients (d33) greater than 1500 pC/N with electromechanical coupling factors higher than 90% along the pseudo-cubic <001> directions. These excellent properties make relaxor-PT single crystals promising candidates for broadband and high sensitivity ultrasonic transducers, sensors and other electromechanical devices. Specifically, certain applications of sensors and transducers, such as accelerometers, vector sensors and non-destructive evaluation (NDE) transducers, require large shear coefficients d15.
It has been reported that rhombohedral single domain PMN-PT crystals poled along their spontaneous polarization direction [111], which may be referred to as having the engineered domain configuration ‘1R’, where ‘1’ represents a single domain crystal and ‘R’ represents the rhombohedral phase, possess high shear values. For these materials, piezoelectric coefficients, d15, and shear coupling factors, k15, are reported to be >2000 pC/N and >90%, respectively, due to the polarization rotation facilitated by the single domain state. Unfortunately, shear piezoelectric coefficients are found to increase significantly with increasing temperature, with more than a 200% change from room temperature to their respective ferroelectric phase transition temperatures. Hence, this strong temperature dependence severely limits their implementation in many electromechanical devices. Furthermore, relaxor-PT single crystals exhibit coercive fields on the order of <2-5 kV/cm, thus limiting applications requiring large AC fields, such as NDE transducers and high power sonar.
What is needed is a piezoelectric single crystal that does not suffer from one or more of the above drawbacks.